Principles and evolution of constructed soils in urban horticulture

Cities are attempting to develop greening and renaturation strategies whose success depends on the quality and functions of the soils supporting the vegetation. However, urban soils are characterized by significant heterogeneity, both vertically and horizontally, due to diverse and contrasting material inputs that lead to particular characteristics: pollution, lower nutrient content, higher pH, higher compaction reducing root and aerial biomass growth, and low organic matter return. To improve the physico-chemical properties of urban soils and ensure sustainable vegetation growth, one solution is to apply the principles of pedological engineering by reconstituting fertile soils using the surface horizon of agricultural plots. However, this practice is currently controversial because of the current decrease in cultivable surfaces and the ensuing threat on food security. An alternative aimed at protecting natural soil resources is to: (1) rehabilitate existing urban soils where possible, or (2) build fertile soils that support several soil functions from materials resulting from urban activity and deconstruction. The “Système d'informations territoriales énergétiques” (SITERRE) program has developed a soil engineering approach to address this issue. It is more particularly based on the formulation of mixtures of technogenic materials to build fertile Technosols that promote plant growth. Research has led to the selection of relevant agronomic fertility indicators based on two types of vegetation model use – Square/Parks and Street Trees. The results demonstrated that it was feasible to build fertile soils from by-products, ready to be planted as soon as they are produced, and guaranteeing safety for human health and the environment.